In soldering electronic components, circuits, equipment and the like, various kinds of fluxes are used together with soldering material so as to improve the efficiency of the soldering operation, to secure the soldered connections and to improve the long-term reliability of the connections while maintaining proper electrical performance. Conventionally, there are three broad categories of flux: (1) natural rosin, (2) activated rosin having a halogenated compound or organic acid activator incorporated with natural rosin, and (3) rosin free flux, generally referred to as water soluble flux.
Natural rosin flux is a stable flux but results in a solid rosin flux residue which, if not completely removed, results in serious contact resistance problems in electronic relays, connectors, gold fingers and printed wiring board (PWB) circuitry. Excessive rosin flux residues also prevent the adherence of protective coatings commonly applied to finished electronic circuit assemblies. It presents few problems with respect to corrosiveness.
Activated rosin flux has a stability similar to natural rosin flux and causes little corrosion at room temperature. Fully activated fluxes have a strong fluxing action at soldering temperatures due to activators, such as an amine hydrochloride, which are typically present in high concentrations such as 1 to 10 weight percent of the resultant flux. However, the fully activated rosin flux has disadvantages in that at soldering temperatures a corrosive gas is produced. Moreover, the residues of the activated rosin combine with moisture to produce corrosive acid. Presently available fluxes containing organic amine hydrohalides in the form of neutral salts such as glutamic acid hydrochloride, either form corrosive metal halides at elevated temperature or the residues thereof combine with moisture at room temperature to form a corrosive acid and thus are used with possible deleterious effect for electrical soldering applications.
There are also mildly activated rosin fluxes such as those taught in U.S. Pat. No. 4,168,996. While these are not corrosive, they still suffer from the same disadvantage of rosin flux in that the flux residue is difficult to remove.
Rosin free fluxes are very useful in removing oxides from the metal surface to be soldered. They generally have the distinct advantage of being relatively easily removed from the device by simple washing with a suitable solvent, alkaline detergent or water. However, there are disdavantages to their use because they frequently contain either strong acids, such as hyrochloric acid or strong organic acids, or an inorganic salt which hydrolyzes in water to give an acid reaction. Therefore, they are apt to destroy metallic material or to leave residues which corrode the soldered parts after soldering, thereby resulting in decreased reliability of the soldered parts with respect to electrical and mechanical properties.
Furthermore, some rosin free fluxes comprise polyethylene glycol and/or it derivatives as a flux vehicle. It has been found that such fluxes interact with the polymeric surfaces of electronic devices, e.g., in printed wiring boards, to modify them so that they become more conductive, thereby increasing still further the chances of device malfunctioning. The change in substrate surface quality with regard to conduction is measurable as a decrease in insulation resistance.
The above-mentioned disadvantages have been substantially alleviated by a rosin free, water soluble flux as set forth in U.S. Pat. No. 4,342,607 when employing low or medium temperature soldering operations, e.g., 350.degree.-500.degree. F. However, when the soldering operation is a higher temperature operation or when greater flux activation is necessary such as in I.R. solder reflow or hot gas leveling techniques, greater thermal stability and/or flux activation is often required than is available from the fluxes taught previously. This high thermal stress exerted on the printed wiring board material makes this process particularly sensitive to the proper choice of the fusing fluid. Because typical polyethylene glycol based fusing fluids show a particular tendency to lower the insulation resistance of the printed wiring product at such high temperatures, there is an obvious need for a high quality and electrically reliable (insulation resistance) fusing fluid.